Transistors are an essential component in modern mobile communications devices. Specifically, transistors play a vital role in the transmission and reception of radio frequency (RF) signals in the front end of a mobile communications device. Due to the decreasing form factor of mobile communications devices, the desire for a longer battery life, and support for an increasing number of stringent wireless communications standards, there is an ongoing need for smaller, more efficient transistor devices with improved performance characteristics.
As will be appreciated by those of ordinary skill in the art, one way to improve the performance of a transistor device operating at high frequencies (e.g., radio frequencies) is by using a heterojunction bipolar transistor. At high frequencies, heterojunction bipolar transistors offer many performance advantages over homojunction bipolar transistors. The performance advantages offered by heterojunction bipolar transistors primarily arise due to a wider energy bandgap in the material of the emitter of the device as compared to the energy bandgap in the material of the base of the device. The wider energy bandgap of the emitter material allows for many parameters dictating the performance of the device to be optimized for high frequencies without degrading the current gain of the device.
Equation 1 shows that the current gain of a heterojunction bipolar transistor has an exponential dependence on the difference in energy bandgap between the emitter and base of the device:
                    β        =                                                            D                nb                            *                              x                e                            *                              N                e                                                                    D                pe                            *                              X                b                            *                              N                b                                              *                      e                          Δ              ⁢                                                          ⁢                                                E                  g                                /                kT                                                                        (        1        )            where β is the current gain of the heterojunction bipolar transistor, Dnb and Dpe are the diffusion constants of electrons in the base and holes in the emitter, respectively, Xe and Xb are the emitter thickness and base thickness, respectively, Ne is the n-type emitter doping concentration, Nb is the p-type base doping concentration, ΔEg is the energy bandgap difference between the emitter and the base, k is the Boltzmann constant, and T is the temperature. The exponential dependence of the current gain (β) of the device on the difference in energy bandgap between the emitter and base allows the doping level of the material used in the base of the device (Nb) to be maximized and the doping level of the material used in the emitter of the device (Ne) to be reduced without pushing the current gain (β) of the device to un-usable levels. The more flexible doping concentrations afforded by using a heterojunction bipolar transistor allow for a lower base resistance and base-emitter capacitance. Further, the high doping concentrations allowed for the base material result in a decreased base thickness and electron transit time. The lower base resistance and base-emitter capacitance offered by heterojunction bipolar transistors provide performance improvements at high frequencies.
One way to further improve the performance of the heterojunction bipolar transistor is by altering the base-collector capacitance and the base resistance of the device. The base-collector capacitance has been shown to significantly improve the small signal gain of a heterojunction bipolar transistor device. This small signal gain improvement can translate into large signal gain improvements in a power amplifier. Therefore, minimizing the base-collector capacitance is critical for improving the large signal gain in both linear and saturated power amplifiers. Further, minimizing the base resistance will also improve the large signal gain of both linear and saturated power amplifiers. One such metric to gauge the small signal improvement is the maximum frequency of oscillation. Other small signal metrics such as maximum available gain and maximum stable gain are also important to measure device improvements through base-collector capacitance and base resistance changes. Equation 2 shows the dependence of the maximum frequency of oscillation for a heterojunction bipolar device on the base-collector capacitance and the base resistance:
                              f          max                ≈                              f            t                                              8              ⁢                                                          ⁢              π              *                              R                b                            *                              C                bc                                                                        (        2        )            where fmax is the maximum frequency of oscillation, ft is the cutoff frequency of the device, Rb is the base resistance, and Cbc is the base-collector capacitance. The maximum frequency of oscillation (fmax), in conjunction with small signal gain metrics, provides a rapidly measurable predictor of large signal gain improvements. Accordingly, minimizing the base-collector capacitance (Cbc) and the base resistance (Rb) of the device play a crucial role in improving the performance of a heterojunction bipolar transistor.
The geometric layout of a heterojunction bipolar transistor has been shown to have significant effects on both the base-collector capacitance and the base resistance of the device. Two quantities can be defined in order to compare the potential performance of a particular geometric layout for a heterojunction bipolar transistor: the emitter area to base-collector junction area ratio (Ae/Abp), and the emitter perimeter to base-contact perimeter ratio (Pe/Pbc). The emitter area to base-collector junction area ratio (Ae/Abp) compares the area of the emitter to the area of the junction between the collector and the base. A larger ratio of emitter area to base-collector pedestal area (Ae/Abp), generally results in a lower base-collector capacitance (Cbc). Ideally, the Ae/Abp ratio would be one, but this is not practically achievable. Conversely, a smaller ratio of emitter perimeter to base contact perimeter (Pe/Pbc), indicates a lower base resistance (Rb). To determine the base resistance, analytical calculations can be done using the heterojunction bipolar transistor geometry and the specific base material properties and base metal contact resistance. However, the emitter perimeter to base contact perimeter ratio is a straightforward and quick calculation to compare the trends in base resistance with geometry. As will be appreciated by those of ordinary skill in the art, these quantities are often provided as a trade-off, and therefore decreasing the base-collector capacitance of the device often comes at the expense of an increased base resistance, and vice-versa. Accordingly, the geometric configuration of a heterojunction bipolar device may dictate the performance of the device.
In addition to affecting the base-collector capacitance and base resistance of a heterojunction bipolar transistor, the geometric layout also significantly affects the thermal properties of the device. For example, the geometric layout of a heterojunction bipolar transistor may affect the heat dissipation characteristics of the emitter of the device. The heat dissipation characteristics of the emitter of the device may become increasingly important as current through the device is increased. Increased current through the heterojunction bipolar transistor may cause excessive heat to accumulate in the emitter, leading to decreased performance and damage to the device. Accordingly, the geometric configuration of a heterojunction bipolar transistor may also dictate the performance of the device in this manner.
FIG. 1 shows a three-dimensional representation of a conventional heterojunction bipolar transistor 10 including a “bar” geometric configuration. The conventional heterojunction bipolar transistor includes a sub-collector 12, a base mesa 14 on a surface of the sub-collector 12, one or more collector contacts 16 adjacent to the base mesa 14 on the surface of the sub-collector 12, a base contact 20 on a surface of the base mesa 14 opposite the sub-collector 12, and an emitter assembly 22 on the surface of the base mesa 14 opposite the sub-collector 12. The base mesa 14 includes a collector layer 17 and a base layer 18.
As shown in FIG. 1, the base mesa 14 is a pyramidal shape including a flat rectangular surface opposite the sub-collector 12. The base contact 20 of the conventional heterojunction bipolar transistor is a “U” shape that encloses the rectangular emitter assembly 22 on three sides. The emitter assembly 22 includes an emitter layer 24, an emitter cap layer 26, and an emitter contact 28. As discussed above, the geometric configuration of the base contact 20 and the emitter assembly 22 may significantly affect the performance characteristics of the conventional heterojunction bipolar transistor 10. Accordingly, several similar “bar” geometric configurations for the base contact 20 and the emitter assembly 22 are commonly employed to improve the performance of the device, as shown in FIGS. 2A-2C.
FIG. 2A shows a two-dimensional representation of the base contact 20 and the emitter assembly 22 of FIG. 1, wherein the base contact 20 surrounds the emitter assembly 22 on three sides. FIG. 2B shows a similar layout to that of FIG. 2A, except the emitter assembly 22 includes two rectangular portions, each rectangular portion surrounded on three sides by an “E” shaped base contact 20. Finally, FIG. 2C shows an emitter assembly 22 including two rectangular portions that are surrounded on two sides by a “T” shaped base contact 20. While the “bar” geometric configurations shown in FIGS. 2A and 2B offer a low emitter perimeter to base contact perimeter ratio (Pe/Pbc), resulting in a relatively small base resistance (Rb), the emitter area to base-collector junction area ratio (Ae/Abp) is also low, resulting in an undesirably large base-collector capacitance (Cbc). Comparing FIGS. 2B and 2C with equal total emitter area, FIG. 2B will have a lower base-collector junction area ratio but a lower emitter perimeter to base-contact perimeter ratio. For device parameters, FIG. 2B would have a larger base-collector capacitance but smaller base resistance. These figures illustrate some of the common heterojunction bipolar transistor geometric trade-offs.
FIG. 3 shows a three-dimensional representation of a conventional heterojunction bipolar transistor 30 including an “annular” geometric configuration. The conventional heterojunction bipolar transistor 30 includes a sub-collector 32, a base mesa 34 on a surface of the sub-collector 32, one or more collector contacts 36 adjacent to the base mesa 34 on the surface of the sub-collector 32, a base contact 40 on a surface of the base mesa 34 opposite the sub-collector 32, and an emitter assembly 42 on the surface of the base mesa 34 opposite the sub-collector 32. The base mesa 34 includes a collector layer 37 and a base layer 38.
As shown in FIG. 3, the base mesa 34 is a tapered cylinder including a flat circular surface opposite the sub-collector 32. The base contact 40 of the conventional heterojunction bipolar transistor 30 is a circular shape, which is substantially enclosed by the ring-shaped emitter assembly 42 with a small opening. The emitter assembly 42 includes an emitter layer 44, an emitter cap layer 46, and an emitter contact 48.
FIG. 4 shows a two-dimensional representation of the base contact 40 and the emitter assembly 42 of FIG. 3, wherein the circular base contact 40 is substantially surrounded by the ring-shaped emitter assembly 42 with a small opening. While the “annular” layout shown in FIGS. 3 and 4 offers a high emitter area to base-collector junction area ratio (Ae/Abp), resulting in a relatively small base-collector capacitance (Cbc), the emitter perimeter to base contact perimeter ratio (Pe/Pbc) is also high, resulting in an undesirably large base resistance (Rb).
FIG. 5 shows a three-dimensional representation of a conventional heterojunction bipolar transistor 50 including a “meander” geometric configuration. The conventional heterojunction bipolar transistor 50 includes a sub-collector 52, a base mesa 54 on a surface of the sub-collector 52, one or more collector contacts 56 adjacent to the base mesa 54 on the surface of the sub-collector 52, a base contact 60 on a surface of the base mesa 54 opposite the sub-collector 52, and an emitter assembly 62 on the surface of the base mesa 54 opposite the sub-collector 52. The base mesa 54 includes a collector layer 57 and a base layer 58.
As shown in FIG. 5, the base mesa 54 is a pyramid shape including a flat rectangular surface opposite the sub-collector 52. The base contact 60 of the conventional heterojunction bipolar transistor 50 includes a central rectangular base connecting several rectangular fingers, which are laterally separated from one another. The emitter assembly 62 is formed around the base contact 60 with a rectangular outer edge. The emitter assembly 62 includes an emitter layer 64, an emitter cap layer 66, and an emitter contact 68.
FIG. 6 shows a two-dimensional representation of the base contact 60 and the emitter assembly 62 of FIG. 5, wherein the base contact 60 is substantially surrounded by the emitter assembly 62. While the “meander” layout shown in FIGS. 5 and 6 offers a relatively moderate base emitter perimeter to base contact perimeter ratio (Pe/Pbc) and emitter area to base-collector junction area ratio (Ae/Abp), resulting in a moderate base resistance (Rb) and base-collector capacitance (Cbc), the performance of the device still has room for improvement.
The geometric configurations described above may offer some performance improvements for a heterojunction bipolar transistor device, however, there is a need for a heterojunction bipolar transistor device with further improved performance, including a better trade-off between the base-collector capacitance and base resistance.